I'm skeptical on most internal filters that include a HEPA filter. I don't think those little fans have enough CFM and static pressure that they can push enough air through both the carbon pellets and the HEPA filter so both they can effectively do their job. The thing with VOCs is you can smell them and know that the filter is or isn't doing its job. Not so with microparticles, for which you need to get a sensor to measure them. And, of course, even sealed printers leak air (I'm looking at you, default Voron split door).
My Trident has a Nevermore V5 Duo for VOCs only. I picked up a nice big Honeywell air clearer with a monster HEPA filter in it that turns over the air like crazy to deal with microparticles. As a bonus, I can use the Honeywell for those times like forest fire season when the air quality is bad.
I don't really know if the HEPA is doing its thing, but I got higher static pressure GDSTIME 5015 fans and can feel the airflow out the other end, past both the HEPA and carbon. As far as visual 'evidence', I can only go off the HEPA filter having turned pink after a printing with a lot of red filament. I mostly just keep it in because the pack of 10 filters was cheap, and the carbon is still doing its job despite the HEPA.
The HEPA probably isn't doing that much, and you're impacting the ability of the carbon to do its job and remove VOCs. I didn't want to play too fast & loose with microparticles given that they are really quite nasty little buggers.
Is there a reason you'd use HEPA instead of having a bigger MERV filter and relying on multiple passes to get the air clean? I thought MERV filters were a better fit if you're recirculating air using a small blower and HEPA does a better job if you can support the pressure drop and absolutely need the particles to be filtered on the first pass.
You can indeed take this route [0] and even DIY something with a box fan. But I think the jury is a bit out on this. Quote from that link below. You also need to move that much more air to get the multiple passes required and that can get pretty noisy.
> They claim that around 90% of particles sized 0.3 microns are larger [for a MERV] are eliminated in a single pass. That’s good, but not totally reassuring. The question is, does it remove 99% in two passes? If 90% of the particles in the ambient air were large and the filter only catches large particles, then additional passes would never get rid of the most dangerous small particles. This is why I trust HEPA filters a bit more: since they remove almost all particles in one pass, I’m confident they should remove almost all particles eventually.
Also, there is the idea that HEPA filters act like a sieve. They don't. This article from the same guy is a great read [1]. So basically, multiple passes for the win.
> Air filters easily catch both large and small particles. It’s the intermediate regime where things are hard. The size where the filter performs worst is called the Most Penetrating Particle Size (MPPS). Typically this is around 0.15 microns.
My Trident has a Nevermore V5 Duo for VOCs only. I picked up a nice big Honeywell air clearer with a monster HEPA filter in it that turns over the air like crazy to deal with microparticles. As a bonus, I can use the Honeywell for those times like forest fire season when the air quality is bad.